This invention relates to a d.c. arc welder which can selectively use two kinds of a.c. power supply of relatively high voltage such as 400 volts and relatively low voltage of about a half thereof such as 200 volts and, especially, to an improved d.c. arc welder which can automatically follow such a power supply as an engine generator whose output voltage gradually rises at the time of starting operation.
A method of responding to such two kinds of supply voltage as above is to provide the primary winding of an input transformer with a center tap and use it at the time of low voltage, as described in Japanese patent opening No. S56-80373, for example. In this method, however, it cannot be expected to reduce the size and weight of the welder, since the input transformer itself has large size and weight. In another method using so-called invertorized configuration in which a.c. power obtained by rectifying an input a.c. power is converted into a high frequency a.c. power by an invertor and it is reduced in voltage by an output transformer which is small in both size and weight and then rectified, as disclosed, for example, in Japanese utility model opening No. H1-151975, a regular rectifier circuit and a voltage doubling rectifier circuit are disposed in the input rectifier to use the input terminals of the latter at the time of low voltage. In these prior art devices, however, it is impossible to use the same input terminals for two kinds of supply voltage in common.
The inventors of this application proposed an invertorized d.c. arc welder which can automatically respond to two kinds of supply voltage without any manual operation in U.S. patent application Ser. No. 07/962,923 and British patent application No. 9217585.0. This welder was provided with a pair of smoothing capacitors for smoothing the output of an input rectifier and a pair of invertors driven by the voltages across the respective capacitors, and arranged to maintain the input voltages of the invertors constant regardless of the input voltage by automatically switching these capacitors in series or parallel between the output terminals of the input rectifier in accordance with the input voltage.
However, this device had such a feature in that it could not effect the above-mentioned switching operation so long as it did not return to its initial condition of zero input, when it turned from the low voltage input to the high voltage input, as described below. Though this feature caused no problem at the time of using constant voltage supplies such as commercial a.c. supplies of 400 and 200 volts since the zero input state happened always at the time of switching them, it caused such a problem in that the above-mentioned switching operation did not take place at the time of using as the power supply an engine generator which was frequently utilized in the mountains or islands having no commercial power supply to be utilized, since its output voltage rises gradually at the time of starting operation.
Accordingly, an object of this invention is to provide an improved arc welder which enables smooth switching operation even when the input voltage varies continuously from a low voltage to a high voltage as described above.